<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <script src="./lib/vue-2.4.0.js"></script>
</head>
<body>
    <div id="app">
        <input type="text" v-model="firstname">+
        <input type="text" v-model="lastname" >=
        <input type="text" v-model="fullname">
    </div>
    <script>
        new Vue({
            el:"#app",
            data:{
                firstname:"",
                lastname:"",
                fullname:""
            },
            methods:{},
            watch:{
                'firstname':function(newval,oldval){
                    this.fullname = this.firstname + this.lastname
                    console.log(newval+"-----"+oldval)
                }
            }
        })
    </script>
</body>
</html>